1. Field of the Invention
This invention concerns a telephone line circuit and, more particularly, a detection device for detecting the "off hook" status of the telephone set during ringing.
2. Description of the Prior Art
In present telephone installations, a subscriber is called by sending an a.c. current through the line from the telephone exchange, which current operates the bells of the subscriber's telephone set. This current is generated from a relatively high a.c. voltage supply (several tens of volts), and with a frequency which varies according to the countries (in the range of several tens of hertz). When, after ringing, the subscriber removes his handset, the loop circuit of this set is closed and a d.c. current is superimposed on the a.c. ringing current. It is, of course, necessary to detect when the handset has been removed; in addition, this detection must be carried out as fast as possible so as to prevent the subscriber from hearing the ringing frequency in his earpiece.
Detection of the d.c. loop current when the ringing current is being sent is a particularly delicate operation in view of the high voltage and low frequency of the ringing current.
The prior art devices to stop sending the ringing current are, generally, comprised of relays which are conceived so as to be as insensitive to the ringing current as possible and which are implemented with contacts which stop the sending of the ringing current as soon as the d.c. current is operated. Such relays are cumbersome, expensive and very difficult to adjust. In addition, they are not readily compatible with modern electronic component line circuits.
Other devices are also well-known in prior art, which are utilized to stop sending the ringing current with the help of threshold detectors, i.e., which are adapted to make discrimination between the presence of the ringing current, alone or the presence of both ringing current and loop current, simultaneously. It is, unfortunately, difficult to have these circuits insensitive to the noise signals which, then, simulate the loop current, such as, for instance, the longitudinal noises.
More generally, most of the prior art devices have made use of analog techniques in that their operations are essentially based upon comparative electrical measurements such as current or voltage, so that such devices can be utilized only under well-known conditions.